diff --git a/docs/ISSUES.md b/docs/ISSUES.md
index a7da1c46..64e04419 100644
--- a/docs/ISSUES.md
+++ b/docs/ISSUES.md
@@ -74,14 +74,20 @@ above the jam (more 38.7° treads → a flat porch at Z 63.6) — so it is ONE l
 not a general stair failure. Classic **#137 / building-collision-mesh junction-anomaly family**
 (TS-4 sliding-normal-provenance), stair edition.
 
-**Open question (decides mesh-artifact vs faithful):** is that 52° face **real dat geometry**
-of object `0xf6822103` or a **mesh-generation artifact** at a BSP-leaf / sub-object junction
-on our side? The capture only records treads the player STOOD on, so it can't show the jam
-face's own extent or the tread directly above it. Decisive next read: dump `0xf6822103`'s
-collision faces (`ACDREAM_DUMP_GFXOBJS`, or resolve its GfxObj/Setup and decode the BSP) and
-check for a spurious ~52° face where a 38.7° continuation should be. If the step to the next
-tread is ≤ 0.6 m (the step-up budget; it looks ~0.4 m), step-up SHOULD lift the player → bug
-in step-up-vs-wall classification for a just-over-walkable face.
+**RESOLVED (artifact, not geometry) — `ACDREAM_PROBE_BUILDING=1` hit-poly capture:** at the
+jam, ALL 156 building collisions are ONE polygon — a real, walkable **38.7° tread quad**
+(`gfxObj=0x01000AC5`, `obj=0xF6822103`, `partIdx=3`, local plane `(-0.625,0,0.781)`; world
+verts X 131.25–132.75, Y 77.00–77.50, Z 60.62–61.01). **There is NO riser / wall polygon in
+the collision mesh at all.** The 52° "wall" normal from the resolve capture (`(0,0.78,0.62)`)
+is that tread normal **rotated exactly 90°** (perpendicular; dot ≈ 0) — it does not exist in
+the dat. So the block is a **synthetic sliding normal fabricated by our collision RESPONSE**,
+not real geometry and not a step-height/budget issue: the foot sphere straddles a convex tread
+edge (near a building-part seam — `partIdx=3`, neighbouring `gfxObj=0x01000ACA` present), hits
+the lower tread poly, and the slide/validate chain invents a perpendicular normal, flattens it
+to horizontal `(0,1,0)`, and slides the player sideways. **Confirmed #137 / TS-4
+sliding-normal-provenance family** (phantom wall at a convex edge/seam), stair edition. Fix
+lives in that family (where the perpendicular normal is fabricated / how a convex walkable
+tread edge is handled) — touches the frozen collision internals → brainstorm gate first.
 
 **Files:** `src/AcDream.Core/Physics/TransitionTypes.cs` (step-up / slide classification, the
 sliding-normal horizontalisation); building-collision mesh generation (BSP faces for